Break out AKS cluster side-effect resource creation

[hasheddan]

What problem are you facing?

As part of the effort for crossplane/crossplane#617, new network resources were introduced to allow connectivity between an AKS cluster and a MySQL server instance (PR: crossplane/crossplane#721, Issue: crossplane/crossplane#647). For example, the MysqlServerVirtualNetworkRule, which allows for connectivity between the AKS subnet and the MysqlServer instance, is implemented as a separate kind with its own controller. This is our long-term goal with all resource types.

However, there are some "sub-resources" that are still being created in the controllers of their "parents". For instance, an AKSCluster creates and Azure AD application and a corresponding service principal before actually creating the AKS cluster with those resources.

How could Crossplane help solve your problem?

While creating sub-resources in the parent controller makes for a more streamlined user experience, it obscures resources and provisioning details. In the long-term, we would like to represent every resource type as its own kind and corresponding controller(s). We will also want to solve the dependency model in Crossplane along with this effort as outlined in crossplane/crossplane#707 and crossplane/crossplane#708.